1. Field of the Invention
The present invention is directed to a wireless communication system and, more specifically, to an Orthogonal Frequency Division Multiple Access (OFDMA) communication system, in light of the development of the 3nd Generation Partnership Project (3GPP) Evolved Universal Terrestrial Radio Access (E-UTRA) Long Term Evolution (LTE).
2. Description of the Art
A User Equipment (UE), also commonly referred to as a terminal or a mobile station, may be fixed or mobile and may be a wireless device, a cellular phone, a personal computer device, a wireless modem card, etc. A Node B (or base station) is generally a fixed station and may also be referred to as a Base Transceiver System (BTS), an access point, or some other terminology.
Several types of signals should be supported for the proper functionality of a communication system. The DownLink (DL) signals consist of data signals, control signals, and reference signals (also known as pilot signals). The data signals carry the information content and can be conveyed from the serving Node B to UEs through a Physical Downlink Shared CHannel (PDSCH). The control signals may be of broadcast or UE-specific. Broadcast control signals convey system information to all UEs. UE-specific control signals convey information related to the scheduling of data signal transmissions from the serving Node B to a UE or from a UE to the serving Node B. The signal transmissions from UEs to a serving Node B occur in the UpLink (UL) of the communication system. The transmission of UE-specific control signals from the serving Node B to UEs is assumed to be through a Physical Downlink Control CHannel (PDCCH).
The DL Reference Signals (RS) can serve for the UEs to perform multiple functions, as known in the art, such as: channel estimation in order to perform demodulation of data signals or control signals; phase reference for Multiple-Input Multiple Output (MIMO) or beam-forming reception; measurements assisting in a cell search and a handover; or Channel Quality Indication (CQI) measurements for link adaptation and channel-dependent scheduling.
The DL RS transmission can have certain characteristics including: time multiplexed (transmitted only during certain Orthogonal Frequency Division Multiplexing (OFDM) symbols); scattered (having a pattern in both the time and frequency domains); common (can be received by all UEs in a serving Node B); dedicated (can be received only by one or a few UEs in a serving Node B); or multiple antennas (in support of MIMO, beam-forming, or transmission (TX) diversity).
An exemplary structure for a Common RS (CRS) transmitted from four antennas of a serving Node B is shown in FIG. 1. FIG. 1 corresponds to one of the structures used in the 3GPP E-UTRA LTE. The DL data packet transmission time unit is assumed to be a sub-frame comprising 14 OFDM symbols 110. Each OFDM symbol is transmitted over an operating BandWidth (BW) comprising OFDM sub-carriers 120 or Resource Elements (REs). Four Node B transmission antennas are assumed. The DL RS from antenna 1, antenna 2, antenna 3, and antenna 4 is respectively denoted as RS1 130, RS2 140, RS3 150, and RS4 160. Each RS has a scattered structure over the DL sub-frame. If only two Node B antennas exist, the corresponding sub-carriers occupied by the RS for Node B antennas 3 and 4 may be used for the transmission of control or data signals or simply left empty. The same applies for the sub-carriers occupied by the RS for antenna 2 if only one antenna exists. The time density of RS1 and RS2 is twice the time density of RS3 and RS4 as the frequency density is the same for all RSs. The former RSs exist in 4 OFDM symbols while the latter RSs exist in 2 OFDM symbols. The rationale for such a non-uniformity is that the use of the third and fourth antennas is typically associated with low to moderate UE velocities, such as, for example, up to 200 Kilometers per hour, and the time density of the respective RS can be decreased but remain adequate to capture the time variations of the channel medium for typical carrier frequencies while the corresponding RS overhead from Node B antennas 3 and 4 becomes half that from Node B antennas 1 and 2.
The RS structures illustrated in FIG. 1 correspond to the CRS which substantially occupies the entire operating BW as opposed to the UE-Dedicated RS (DRS) which typically occupies only the BW where a UE is scheduled to receive DL data packet reception in the PDSCH. This enables the CRS to be used for the reception of signals with frequency diverse transmission, such as, for example, control signals, for CQI measurements, or for cell search and handover measurements. However, if the RS is intended to be used only for providing a phase reference for beam-forming or MIMO, a DRS transmitted over the PDSCH data packet transmission BW to a UE suffices. In FIG. 1 for the PDCCH and PDSCH multiplexing, the PDCCH 170 occupies the first N OFDM symbols while the remaining 14-N OFDM symbols are typically assigned to PDSCH transmission 180 but may occasionally also contain transmission of synchronization and broadcast channels.
An OFDM transmitter is illustrated in FIG. 2. The information data 210 is first encoded and interleaved by coding and interleaving unit 220, for example, using turbo encoding and block interleaving. The data is then modulated in modulator 230, for example, using QPSK, QAM16, or QAM64 modulation. A Serial to Parallel (S/P) conversion is applied to generate M modulation symbols in S/P converter 1:M 240 which are subsequently provided to an IFFT unit 250 which effectively produces a time superposition of M orthogonal narrowband sub-carriers. The M-point time domain blocks obtained from the IFFT unit 250 are then serialized Parallel to Serial (P/S) converted M:1 260 to create a time domain OFDM signal 270. The RS transmission can be viewed as a non-modulated data transmission. Additional functionalities, such as data scrambling, cyclic prefix insertion, time windowing, filtering, and others are well known in the art and are omitted for clarity.
The reverse functions are performed at the OFDM receiver as illustrated in FIG. 3. The received OFDM signal 310 is provided to a serial to parallel converter 320 to generate M received signal samples which are then provided to an FFT unit 330, and after the output of the FFT unit 330 is serialized in P/S converter 340, the signal is provided to demodulator 350 and decoding and deinterleaving unit 360 to produce decoded data. Similarly to the OFDM transmitter structure in FIG. 2, well known in the art functionalities such as filtering, time-windowing, cyclic prefix removal, and de-scrambling are not shown for clarity. Also, receiver operations such as channel estimation using the RS are also omitted for clarity.
The total operating BW may consist of elementary scheduling units, referred to as Physical Resource Blocks (PRBs). For example, a PRB may consist of 12 consecutive sub-carriers. This allows the serving Node B to configure, through the PDCCH, multiple UEs to simultaneously transmit or receive data packets in the UL or DL by assigning different PRBs for the packet transmission or reception from or to each UE. For the DL, this concept is illustrated in FIG. 4 where five out of seven UEs are scheduled to receive data in one sub-frame over 8 PRBs 410. UE1 420, UE2 430, UE4 440, UE5 450, and UE7 460, are scheduled for PDSCH reception in one or more PRBs while UE3 470 and UE6 480 are not scheduled for any PDSCH reception during the reference sub-frame 490. The allocation of PRBs may or may not be contiguous in the frequency domain and a UE may be allocated an arbitrary number of PRBs (up to a maximum number as determined by the operating BW and the PRB size).
The Node B scheduler can select the PRBs used to transmit the data packet to a scheduled UE based on the CQI feedback from the scheduled UE over a set of PRBs. The CQI feedback is typically a Signal-to-Interference and Noise Ratio (SINR) estimate over a set of PRBs as illustrated in FIG. 5. The Node B scheduler can use this information to schedule PDSCH transmissions to UEs in the PRBs where the SINR is the highest, thereby maximizing the system throughput. In FIG. 5, the SINR 501 of UE1, the SINR 502 of UE2 and the SINR 503 of UE3 are maximized respectively over the PRB sets 504, 506, and 505 and the corresponding PDSCH transmissions can be over these PRB sets.
If the set of PRBs is a set corresponding to the entire operating BW, a RS for the respective Node B transmission antenna port is needed over the operating BW to obtain the CQI estimate and, as previously mentioned, requires the use of a CRS. For the sub-frame structure and the RS structure in FIG. 1, the total RS overhead from four Node B transmission antennas is equal to 14.3% of the total overhead, which is significant but not unacceptably large.
The maximum and average supportable data rates in a communication system depend, among other factors, on the number of transmission antennas. In order to increase these data rate metrics, and thereby more effectively utilizing the BW resource, additional antennas are often required. To enable gains in system throughput and peak data rates afforded by increasing the number of transmission antennas to be realized in practice, it is essential to avoid a substantial increase in the total RS overhead as required to support signal transmission from the additional antennas. For example, for eight Node B transmission antennas, even if antennas 5-8 employed the RS structure with reduced time density as antennas 3 and 4 in FIG. 1, the total RS overhead would be 23.8% of the total overhead, which is unacceptably large.
Additionally, it is often desirable to support PDSCH transmission to UEs with different capabilities. For example, some UEs may be able to receive PDSCH transmissions from a maximum of only four Node B antennas (legacy UEs) while other UEs may be able to receive PDSCH transmissions from a maximum of eight Node B antennas (non-legacy UEs). Support for RS transmitted from eight Node B antennas should not conflict with the capability of legacy UEs to receive PDSCH transmitted from a maximum of four Node B antennas without requiring additional receiver operations.
Therefore, there is a need to avoid proportionally increasing the RS overhead as the number of Node B transmission antennas increases.
There is another need to support RS transmissions for providing reliable data scheduling at the Node B, by enabling the UEs to provide the appropriate CQI feedback, and to enable reliable signal reception at UEs as the number of Node B transmission antennas increases.
There is yet another need to support RS transmissions from a number of Node B antennas without affecting the signal processing at UE receivers capable of processing only signals transmitted from a smaller number of Node B antennas.